In a process control system adopting field-buses by a two-wire bus-feed type digital communication, such as Foundation Fieldbus FF-Hi, PROFIBUS-PA, and so forth, for exchanging input/output of processes for temperature, flow rate, and so forth between fields (operation sites), and a control system, there is a demand for quality control for constantly monitoring communication quality of those field-buses, and notifying a user of deterioration in the communication quality before actual occurrence of a communication trouble so as to enable the user to take necessary countermeasures.
In the past, input/output of information was executed by a one-to-one analog signal between a controller and field equipment units such as a thermometer, a pressure transmitter, valve positioner, and so forth, respectively, however, concurrently with arrival of times when a number of pieces of information per one equipment unit can be handled in digital communication through adoption of the field-buses, it has become possible to handle a plurality of equipment units on one signal line (a segment).
For this reason, there has since been adopted a design, for example, as disclosed in the Patent Document 1 whereby field bus equipment units in number as many as possible are interconnected on one segment so as to enhance efficiency in mounting partly because a field-bus communication input-output module is expensive as compared with an analog input-output module.
FIG. 7 is a functional block diagram showing one embodiment of a conventional control system adopting a field-bus. A host unit 1 communicates with a controller 3 via a control bus 2. The controller 3 adopts a multi-drop system configuration for communication with a plurality of equipment units 6a, 6b, . . . , 6n, connected to a field-bus 5 making up one segment, via a field-bus communication input-output module 4 to thereby execute control of those equipment units.
FIGS. 8(A) to 8(C) are waveform charts of frames on a field-bus, subjected to effects of a noise. FIG. 8(A) shows a waveform of frames in a state where a noise is not present on the field-bus (or a state of a voltage level within a tolerable range according to the specification). In this case, an interval between communication frames is found substantially uniform according to a field-bus specification.
FIG. 8(B) shows the effect of one-shot noise as overlaid, indicating that if the noise is overlaid in the middle of a frame, destruction occurs to the content of the frame (phenomenon 1) while if the noise is overlaid between the frames, an unstable expansion occurs to an interval between the frames (phenomenon 2.)
FIG. 8(C) shows the effect of a noise minute in amplitude, in as-overlaid state, indicating occurrence of the state of the frame intervals undergoing the unstable expansion (phenomenon 2) as with the case of FIG. 8(B), and a state of incompletion in signal reception action (phenomenon 3).
Upon frequent occurrence of those phenomena 1 to 3, it follows that communication troubles have emerged. Diagnosis on soundness of the relevant segment is made on the basis of the number of times the frame is destructed (CRC Error check).
In a stage before those phenomena occur (a stage where a noise level is still relatively low), a phenomenon of “frame interval time is unstable” as in the case of the phenomenon 3 will come to be observed on the actual bus. The reason for this is because the equipment unit of the field-bus that is about to start transmission recognizes those noises as part of the actual frame, delaying therefore timing for starting transmission in order to comply with a minimum frame-interval time according to the field-bus specification.
FIGS. 9(A) and 9(B) are time charts showing frame-intervals in disorder in a noise environment. FIG. 9(A) shows frame-interval time in a noiseless proper communication environment where frame-intervals are substantially uniform.
Meanwhile, variation occurs to the frame-interval due to delay in the timing for starting transmission owing to the noise as in the case of the phenomenon 2 even though a state of the destruction of the frame has not been reached as yet. FIG. 9 (B) shows a state of the variation occurring to the frame-intervals.
[Patent Document 1] JP 2006-287684A
In the case of input/output function making use of the field-bus, if an improper waveform as a noise is overlaid on the segment, or the equipment unit connected to the field-bus falls in a state of failure, sending out an improper waveform onto the segment, thereby causing occurrence of a communication trouble, this can exert adverse effects on all the other equipment units interconnected on the same segment as shown in FIG. 7, so that there can be a possibility that communication with the equipment units on the same segment is no longer possible at the worst, and a control operation by the controller will be stopped.
In order to preemptively prevent occurrence of such a situation described as above, a segment diagnostic function for diagnosing quality of communication of a segment to thereby give a warning to a user has been developed by respective venders. With the current state of the art, use of “destruction of frame data” as an index of the quality of communication of a field-bus segment is in the mainstream of the art.
However, since the “destruction of frame data” represents a phenomenon substantially at the same level as that for the communication trouble, and a distance from “detection to occurrence of the communication trouble” is very short, there exists a problem in that the destruction of frame data often cannot be used except for in a situation for finding the cause of the communication trouble, and therefore, cannot be used in techniques for predictive diagnosis on the communication trouble.